In the human eye, the final stage of image processing in the retina is performed by retinal ganglion cells. The axons of the ganglion cells project out of the eye to form the optic nerve. Glaucoma, which produces irreversible blindness if not treated early enough, destroys these ganglion Cells. A typical "early" sign of glaucoma is the loss of a portion of the peripheral visual field, referred to as "scotoma". Unfortunately, by the time a scotoma is detected, the disease has reached a stage where treatment can at best prevent further irreversible blindness.
A symptom of glaucoma is an increase in the patient's intraocular tension. Thus one approach that is used to screen patients for glaucoma at an earlier stage than when a scotoma has developed is to test the intraocular tension of a patient. The measurement of intraocular tension, however, usually involves the use of drugs, is time consuming, and is unpleasant for the patient.
Moreover, some glaucoma patients do not exhibit intraocular tensions above about 21 mm of mercury (such patients have what is called "normal-tension glaucoma"). Thus, testing the intraocular tension of a person is not a reliable method for the early detection of glaucoma.
Another proposal for the early detection of glaucoma has involved the assessment of colour vision defects. Simple tests of colour vision defects, however, have shown a lack of correlation between the defects noted and the presence of optic disc cupping. More complex tests of colour vision defects, which involve anomaloscopy, ere too difficult for clinical use. Moreover, those tests cannot differentiate between colour vision defects caused by glaucoma and colour deficits resulting from amblyopia and optic neuritis. In addition, it has been reported that up to 25 per cent of patients who have glaucomatous scotoma exhibit no colour deficit. Thus assessment of colour deficits in a person's vision is not a reliable method of detecting glaucoma in its early stages, even if it should become practical to perform detailed colour vision tests clinically.
Another conventional method for determining whether an eye has been damaged by glaucoma involves the use of perimetry. In a typical perimetric investigation, a patient is presented with a series of small luminous dots projected onto a screen which has been placed in front of the patient. The patient's vision is assessed on the basis of whether the patient testifies to seeing each dot as it is presented. This procedure, being a serial search, is very time consuming and is prone to errors arising from subjects' fallibility. Moreover, this technique is not primarily designed for assessing glaucomatous damage but rather for assessing localised blind spots, which can arise from a variety of retinal and central nervous system disorders that affect vision. In particular, perimetric investigations take no account of the spatial scale of the visual system, for they typically use the same size dots in all parts of the visual field, and thus they do not investigate whether any particular visual subsystem has been damaged by glaucoma.